Subject: Re: CCL:Second negative frequency

From: "Dr. Peter Burger" <[email-masked]>
Date: Tue, 13 Feb 2001 18:10:51 +0100 (CET)

Dear Adel,

> I think your problem is with the numerical derivatives. I think using the
> analytical would solve you problem. You do not you try small basis set

nice try, but in the (yet) absence of this capability in the program
I use, i.e. Turbomole I am left with that problem.

> with analytical or if you can the smallest of your molecules with
> analytical. There are so many paprameters also to vary with the numerical.
> Why do not try to vary these (I really do not think it will help but you
> may try). Why do not you try tighter geometry optimization. I think that

well, sure I do all that kind stuff, tighten SCF-convergence etc., change 
the stepsize of the finite differences but you see, you do not know 
the answer in advance after 6n SCF steps and gradients. In my case I may
have the answer just after 2-10 days etc. depending on the size of the system
 (50-130 atoms 500-1500 bfs) and numbers of CPU I use (though it
runs perfectly parallel, calculations are done on 8-20 CPUs).  Checking
for / fighting the numerical noise is not
just gratifying considering error bars of at least +-20 cm-1 - so should
I stop and relax with a second small imaginary frequency of -20 to -30
cm-1?

Peter

P.S. Also I forget to point out that a number of people seem to be just
     happy and get away with the updated approximate hessian they get
     during the optimization process - is that any better? I do not
     think so.